This invention relates to a PH electrode using an ISFET (ion-sensitive field-effect transistor) which can be used, without modification, with a conventional PH meter using a glass electrode.
FIG. 1 shows a conventional PH measuring circuit using a glass electrode. In FIG. 1, reference numeral 1 designates a PH meter (or a potentiometer); 2, electrolyte; 3, a reference electrode; and 4, a glass electrode. In order to electrically measure the PH value of the electrolyte, the PH meter using the glass electrode 4 has been extensively employed because its PH measurement range is wide and its measurement accuracy is high. The glass electrode 4 is in the form of a ball or a tube (whose diameter is 5 to 10 mm) made of a glass film in which a buffer solution whose PH value is known, and an electrode, are sealed. The potential difference between the glass electrode 4 and the reference electrode 3, which are inserted into the electrolyte 2, is in a certain relation to the PH value of the electrolyte 2. Therefore, the PH value of the electrolyte 2 can be determined by measuring the potential difference with the PH meter.
Shown in FIG. 2 is another conventional PH measuring circuit using an ISFET. In FIG. 2, reference numeral 5 designates the ISFET; and 6, a constant current source. Only recently have ISFETs been developed, and it has been known that the PH value of an electrolyte can be measured with an ISFET. When, under the condition that the ISFET is inserted into the electrolyte 2, a predetermined voltage +V is applied to the source S of the ISFET and current is allowed to flow between the source S and the drain D thereof from the constant current source 6, the value of the voltage will depend on the PH value of the electrolyte 2. Therefore, the PH value of the electrolyte 2 can be measured by providing a voltage between the drain D and the reference electrode 3 as an output V.sub.out. However, in order to measure the PH value with an ISFET, it has been necessary to provide the constant current source (6). Therefore, measurement could not be achieved using the PH measuring circuit with the conventional glass electrode.